capture - SKILL.md Agent Skill

name: capture description: Capture a REAL Claude Code /v1/messages body as a replay fixture, so the HTTP replay arm (.skills/replay) faithfully reproduces Claude Code's cache_control breakpoint structure (a hand-authored fixture makes the 1h-TTL knob look like a no-op). Starts a clawback proxy with --capture-body, drives ONE real claude turn through it via the PTY driver, verifies the dump carries cache_control breakpoints, and promotes it to the fixture path. Spends real Anthropic tokens (exactly one turn; pin a cheap model with --model). Use once before replay-based benchmarking, or to refresh the fixture when Claude Code changes its request shape.

clawback fixture capture

Run .skills/capture/scripts/capture_fixture.sh from the project root. It captures the genuine Claude Code request body end to end:

starts a standalone clawback proxy with --capture-body <tmp> (detached, via .skills/monitor's run_monitor.sh --detach);
drives one real claude turn through it (benchmark/bin/drive_pty.js --turns 1), which points claude at the proxy and types a single prompt;
stops the proxy;
verifies the dump with benchmark/bin/verify_fixture.js (must carry ≥1 cache_control breakpoint, else the 1h-TTL arm is a no-op);
promotes the temp dump to the fixture path at mode 0600.

The captured bytes are the request body only — system prompt, tools, and the one message. The Anthropic API key is not captured (it rides in headers clawback never dumps). Still, treat the fixture as sensitive: it contains your system prompt and tool definitions. It is gitignored-by-convention and excluded from the npm tarball.

Spends real tokens

Step 2 runs the real claude binary against your own Anthropic limits — exactly one turn. Pin a cheap model to keep it trivial:

.skills/capture/scripts/capture_fixture.sh --model claude-haiku-4-5-20251001

Requirements: the claude CLI on PATH, node-pty installed (npm i node-pty), and claude already authenticated.

Usage

# default: capture to benchmark/fixtures/ccode.json on 127.0.0.1:8787
.skills/capture/scripts/capture_fixture.sh --model claude-haiku-4-5-20251001

# custom destination / port / prompt:
.skills/capture/scripts/capture_fixture.sh --out benchmark/fixtures/mywork.json \
  --port 8790 --prompt "Summarize README.md in one line."

Options

--out PATH — fixture destination (default benchmark/fixtures/ccode.json).
--host, --port — proxy bind (default 127.0.0.1:8787).
--model ID — pin claude's model (cost control; forwarded to drive_pty).
--prompt TEXT — the single prompt to type (default: a read-only listing).
--keep-tmp — keep the temp dump even if verification fails (debugging).
everything after -- is forwarded to drive_pty.js.

Why this exists

--capture-body (in src/server.js) dumps the pristine body once, before clawback's strip/1h-rewrite mutations — so the fixture is exactly what Claude Code put on the wire. Replaying it through clawback then exercises the real cache mechanic. See .skills/replay.

After capture

Replay the fixture with .skills/replay, then analyze with .skills/bench and visualize with .skills/plot. For the faithful (non-replay) arm that drives real claude live, see .skills/drive.